1. Field of the Invention
This invention relates to a screening device for bulk materials, having at least one screening floor, and with a plurality of shafts aligned parallel with each other and rotating at the same speed in the same direction. The invention provides screening stars with teeth, projecting radially from shafts where they are mounted, interleaved with the screening stars of the adjacent, parallel shaft.
2. The Prior Art
Screening devices of this type are generally known from European Patent No. EP 0 838 667 A2. Fractions of the bulk material are separated from each other by means of the screening stars installed on a screening floor, where they are mounted with torsional strength on rotating shafts. By means of the spacing present between each two adjacent screening stars, which rotate with their teeth revolving pass each other, these spacings being present due to the fact that the screening stars are mounted next to each other, the screening stars form a latticework of gaps through which the desired grain fraction of the bulk material can be separated from an oversized grain fraction. The spacings available between the screening stars are dimensioned so that the particles of the desired grain fraction drop through the gaps, whereas the particles of the oversized grain fraction are prevented from passing through. By means of the screening stars mounted on, and rotating with the shafts, grain particles are moved across the screening floor and are transported, for example to a collection station.
The grain particles of the oversize grain fraction are moved in this process through the teeth of the screening stars, where they also can drop down and be received in the troughs of the teeth, so that they can be ejected again from these troughs by the centrifugal forces acting on them as the screening stars are rotating. This ejection from the trough of a tooth can be supported by the teeth of a adjacent screening star mounted on a neighboring shaft, so that the front flank, viewed in the direction of rotation of the screening star, is brought close to an oversize grain particle present in the trough of a tooth. The oversized particle is then lifted from the trough of the tooth by the further rotation of the shaft, supporting the screening star, until it is ejected therefrom. With such screening apparatus, it is known that due to the interaction between two teeth of the screening stars, neighboring on one another, a crushing of particles of the oversized grain fraction occurs as well. These particles are jammed between the flanks of a tooth, with the disadvantageous result that the screening stars become clogged up. This clogging of the screening stars, until they are completely blocked, finally leads to a soiling of the screening device. Moreover, the complete closure of the spacing between the screening stars may be caused in disadvantageous ways under certain circumstances, so that the screening efficiency of the practically clogged screening device is reduced, or even completely canceled. Furthermore, pieces of debris getting jammed between the screening stars such as, for example, pieces of wood, nails and rocks may block the rotational motion of individual or several shafts, causing interference with the entire screening device and the screening process. The screening device then has to be shut down, and cleaned in a cumbersome way before it can be restarted.